This invention is directed to the control of a field variable signature through local sensors and active elements by determining a transfer matrix between discrete local and midrange field values. Data bases are accumulated on any platform for a magnetic field due to degaussing coil activation and to various magnetic states at local and midrange sensor sites.
The hulls of seagoing vessels constructed of ferromagnetic materials exhibit both permanent and induced magnetic properties. The magnetic field, or signature, of these material sources is a significant fraction of the Earth's field and is not linearly proportional nor parallel to the Earth's field. Nor is the distribution of magnetized materials homogeneous throughout the vessel. Other factors such as hydrostatic pressures acting on the hull, vibrations imparted to the hull by various machinery, as well as the particular orientation of the vessel with respect to the Earth's magnetic field all contribute to the variability of the magnetic state (the equivalent distributed dipole source strength) of the vessel. Since the presence of a magnetic field around the vessel which is dissimilar to its magnetic environment permits detection of the vessel by, for example, magnetic anomoly detection sensors carried aboard aircraft, a means of nullifying the net magnetization of the vessel, and thus the magnetic signature of the vessel, is especially desirable.
Current methods of determining a vessel's magnetic state and its signature, involve the measurement of the vessel's external magnetic field using a degaussing range each time such knowledge is desired. This costly and time consuming effort is often not a reliable indicator of the vessel's magnetization at a later time because of variations of the magnetic state with time. Further, magnetization of vessel materials is known to vary widely under the influence of pressure, temperature, and external magnetic fields so that onboard measurements and self-monitoring of the vessel's magnetic state is needed to determine the vessel's magnetic state to provide input to a degaussing coil current controller.